Thermal energy scavenger (stress limiter)

ABSTRACT

A thermal energy scavenger assembly including a plurality of temperature-sensitive wires made of material which exhibits shape memory due to a thermoelastic, martensitic phase transformation. The wires are placed in tension between fixed and movable plates which are, in turn, supported by a pair of wheels which are rotatably supported by a housing for rotation about a central axis. A pair of upper and lower cams are fixed to the housing and cam followers react with the respective cams. Each cam transmits forces through a pair of hydraulic pistons. One of the pistons is connected to a movable plate to which one end of the wires are connected whereby a stress is applied to the wires to strain the wires during a first phase and whereby the cam responds to the unstraining of the wires during a second phase. A housing defines fluid compartments through which hot and cold fluid passes and flows radially through the wires whereby the wires become unstrained and shorten in length when subjected to the hot fluid for causing a reaction between the cam followers and the cams to effect rotation of the wheels about the central axis of the assembly, which rotation of the wheels is extracted through beveled gearing. The wires are grouped into a plurality of independent modules with each module having a movable plate, a fixed plate and the associated hydraulic pistons and cam follower. The hydraulic pistons and cam follower of a module are disposed at ends of the wires opposite from the ends of the wires at which the same components of the next adjacent modules are disposed so that the cam followers of alternate modules react with one of the cams and the remaining cam followers of the remaining modules react with the other cam. There is also included stress limiting means in the form of coil springs associated with alternate ends of the wires for limiting the stress or strain in the wires.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a thermal energy scavenger or a thermal energyconverting assembly of the type for converting heat energy intomechanical energy and, more specifically, to such an assembly utilizinga plurality of temperature-sensitive elements made of a material whichexhibits shape memory due to a thermoelastic, martensitic phasetransformation whereby less energy is required to strain the elements ina cold condition than the energy returned when the elements becomeunstrained as they are heated to a higher temperature.

During recent years various materials comprising metal alloys have beendeveloped which have a shape memory characteristic based uponthermoplastic, martensitic phase transformations which are stress orstrain dependent. Basically, such alloys exhibit a stable shape in aphase above a given transition temperature and experience atransformation to a martensitic phase at a temperature below thetransition temperature. The alloys have a much lower effective modulusat the martensitic phase below the transition temperature therebyrequiring a relatively small amount of energy in the form of stress forstraining the alloy when at the lower temperature whereas the alloyprovides much more energy as it unstrains and returns to its originalshape when it reaches a higher temperature above the transitiontemperature. Examples of alloys which have this shape memorycharacteristic are titanium-nickel; copper-aluminum-nickel; copper-zinc;iron-platinum and gold-cadmium.

A discussion of the shape memory characteristics in a number of alloysis set forth in the Journal of Material Science; 1974, Volume 9, Pages15-21 by the authors L. Delaey, R. V. Krishnan and H. Tas. Furtherdiscussions are set forth in Metallurgical Transactions; 1975, Volume6A, Page 29 by H. C. Tong and C. M. Wayman.

Further description of materials having the shape memory characteristicare set forth in U.S. Pat. No. 3,174,851 granted Mar. 23, 1965 toWilliam J. Buehler and Raymond C. Wiley and U.S. Pat. No. 3,558,369granted to F. E. Wang and William J. Buehler on Jan. 26, 1971.

(2) Description of the Prior Art

There have been efforts to utilize these materials, which have shapememory characteristics, in thermal energy converting assemblies and suchassemblies have proved that the materials may be so utilized. Suchassemblies strain the material having the shape memory characteristicand extract energy but have not efficiently utilized the material in themanner in which the material is strained nor maximized the amount ofmaterial strained in a given space. Additionally, the material havingthe shape memory characteristic may be strained to a greater extentwhile below the transition temperature than it may be strained whileabove the transition temperature and the prior assemblies may strain thematerial equally when both below and above the transition temperature.

SUMMARY OF INVENTION

A thermal energy scavenger assembly comprising at least onetemperature-sensitive element made of material which exhibits shapememory due to a thermoelastic, martensitic phase transformation andreaction means reacting with the element for applying a stress to theelement to strain the element during a first phase and for responding tothe unstraining of the element during a second phase with stresslimiting means for limiting the strain of the element during the secondphase whereby the strain upon the element may be greater during thefirst phase than during the second phase.

PRIOR ART STATEMENT

A very basic prior art assembly is disclosed in U.S. Pat. No. 3,403,238granted Sept. 24, 1968 to William J. Buehler and David M. Goldstein,which patent merely discloses the simple concept of placing a shapememory material of nickel-titanium under stress to strain the materialby cantilevered bending or torsional twisting at a relatively lowtemperature and extracting the increased energy resulting from theunstraining or return of the material to its original unbent oruntwisted shape as it reaches a higher temperature. Tests have also beenconducted on an assembly utilizing rods of a material having a shapememory. The rods extend between non-parallel rotating discs so thatsynchronous rotation of the discs increases the distance betweencorresponding points of attachment of the rods on their perimetersduring one-half revolution and decreases the distance between the endsof the rods during the other half revolution whereby the rods arestrained by being placed in tension at a lower temperature and contractto their original length when heated to a higher temperature. Such testswere conducted by the Lawrence Berkley Laboratory of the University ofCalifornia and reported in their report NSF/R Ann/SE/AG550/ FR 75/2entiled "Nitinol Engine Project Test Bed" dated July 31, 1975. The shapememory material utilized in that project was 55-Nitinol from the TimetDivision of the Titanium Corporation of America, Toronto, Ohio with achemical composition of 55.38% nickel; 0.05% iron; 0.004% nitrogen andthe balance titanium.

A further such assembly comprising this material is disclosed in U.S.Pat. No. 3,913,326 granted Oct. 21, 1975 to Ridgway M. Banks. In thisassembly the material is in the form of wires which are in a U shape inthe hot side and are straight or relatively straight in the cold side.Two other assemblies are disclosed in U.S. Pat. No. 4,037,411 grantedJuly 26, 1977 to Peter A. Hochstein. One of the assemblies bends stripsof metal in a cantilevered fashion and the other assembly twists stripsof metal along their length.

In none of the prior art assemblies, however, is there the maximum useof the metal as in accordance with the subject invention wherein theassembly applies a stress to an element of the material having the shapememory characteristic to strain the element during the first phase andfor responding to the unstraining of the element during a second phasewith stress limiting means for limiting the strain of the element duringthe second phase whereby the strain upon the element may be greaterduring the first phase than during the second phase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a preferred embodiment of thesubject invention;

FIG. 2 is a plan view taken substantially along line 2--2 of FIG. 1;

FIG. 3 is a fragmentary cross-sectional view taken substantially alongline 3--3 of FIG. 2;

FIG. 4 is a fragmentary cross-sectional view taken substantially alongline 4--4 of FIG. 3;

FIG. 5 is a top plan view similar to FIG. 2 but showing the assemblypartially broken away and in cross section;

FIG. 6 is a fragmentary cross-sectional view taken substantially alongline 6--6 of FIG. 7;

FIG. 7 is a cross-sectional view partially broken away and in crosssection and taken substantially along line 7--7 of FIG. 3;

FIG. 8 is an enlarged fragmentary view showing one of the plates towhich the elements are attached;

FIG. 9 is an enlarged fragmentary cross-sectional view partially brokenaway and in further cross section of the elements extending between theplates with stress limiting means associated therewith; and

FIG. 10 is a perspective view partially cut away and in cross sectionshowing the preferred embodiment of the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A thermal energy scavenger assembly constructed in accordance with thesubject invention is generally shown at 10, with the combination ofcomponents best illustrated in FIG. 10.

The assembly 10 includes a plurality of temperature-sensitive elementstaking the form of elongated wires 12 having a circular cross sectionwith first and second ends and made of material which exhibits shapememory due to thermoelastic, martensitic phase transformation. Thematerial may be any one of those discussed above requiring less energyfor straining the elements at a relatively cold temperature than may beextracted from the elements as they return to the original unstrainedshape while at a higher temperature, i.e., shape memory characteristic.

The assembly 10 also includes reaction means or cam means comprising apair of first and second or upper and lower, axially spaced cams 14 and16 respectively. The cams 14 and 16 react with the wire elements 12 forapplying a stress to the elements 12 to strain the elements 12 during afirst phase and for responding to the unstraining of the elements 12during a second phase. The phase transformation occurs as the elements12 pass through a transition temperature between a relatively cooltemperature below the transition temperature and a relatively warmtemperature above the transition temperature. The first phase occurswhile the elements are at the relatively cool temperature below thetransition temperature whereas the second phase occurs while theelements are at the relatively warm temperature below the transitiontemperature. The cams 14 and 16 are disposed about a central axis 18.The cam 14 has an axially extending cam surface 20 disposed about theradial periphery thereof and the cam 16 has an axially extending surface22 disposed about the radial periphery thereof.

The assembly 10 also includes carriage means generally shown at 24 forsupporting the wires 12 in parallel relationship to the central axis 18and in parallel relationship with one another for allowing the elementsor wires 12 to be placed in tension while reacting with the cams 14 and16. Said another way, the carriage means 24 positions or places thewires 12 in tension in response to reaction with the cams.

There is also included a support means defined by the housing generallyindicated at 26 which supports the cams 14 and 16 and the carriage means24 for allowing relative movement between the cams 14, 16 and thecarriage means 24 to extract energy as the wires 12 unstrain and shortenin length during the second phase. More specifically, the cams 14 and 16are fixed to the support means defined by the housing 26 and thecarriage means 24 is rotatably supported by the support means defined bythe housing 26 for rotation about the central axis 18. The cams 14 and16 are secured to the inner wall 28 of the housing by being weldedthereto. The cam 14 is an integral part of a cam and bearing supportmember 30 and the cam 16 is an integral part of a cam and bearingsupport member 32. The cam and bearing support member 30 supports thebearing assembly 34. The cam and bearing support member 32 supports abearing assembly 36. The bearing assemblies 34 and 36 rotatably supportthe carriage means 24.

The carriage means 24 includes an upper rotating wheel having an innerhub portion 38 and an outer rim portion 40 with the two portionsinterconnected by circumferentially spaced spokes 42 and a lowerrotating wheel including an inner hub portion 44, an outer rim portion46 with circumferentially spaced spokes 48 interconnecting the portions44 and 46. The wheels are axially spaced from one another along thecentral axis 18 of the assembly. The upper wheel is connected to a ringmember 50 by a plurality of bolts 52 and the ring member 50 is supportedby the bearing 34 for rotatably supporting the upper wheel. The lowerwheel is connected by the bolts 56 to a ring member 54 which, in turn,is supported by the bearing 36 for rotatably supporting the lower wheel.The ring member 54 has a flange with a seal 58 secured thereto.Additionally, seal assemblies 60 are disposed between the respectivewheels and flanges of the cam and bearing support members 30 and 32.

The carriage means 24 includes a plurality of independent modulesdisposed in a circle about the central axis 18, two of which areillustrated in FIG. 4. Each of the modules supports a group of the wires12 so that each group of wires react in unison with one of the cams 14or 16 and independently of the groups of wires 12 of other modules. Eachof the modules includes a cam follower assembly 62 for engaging one ofthe cams 14 or 16. Further, each of the modules includes hydraulic meansdisposed between each of the cam follower assemblies 62 and theassociated group of wires 12 of each module for transferring reactionforces between the cams 14 and 16 and the wires 12 of each groupthereof. The hydraulic means is disposed between the wires 12 and thereaction means defined by the cams 14 and 16 for transferring reactionforces between the reaction means and the wires 12 through a fluid underpressure. The hydraulic means includes a plurality of independenthydraulic packages with each hydraulic package associated with one ofthe modules. The hydraulic package associated with each module includesa first hydraulic chamber 64 disposed in one of the outer rim portions40 or 46 with a first piston 66 movably disposed in each chamber 64 todefine an expandable hydraulic volume 74. Each hydraulic package alsoincludes a second hydraulic chamber 68 disposed in one of the rimportions 40 or 46 and a second piston 70 movably disposed in eachchamber 68 to define a hydraulic volume 76 which may expand andcontract. The chambers 68 are closed by the plugs 72. The expandablevolume 74 defined by the chamber 64 and the piston 66 is in fluidcommunication by a hydraulic line (not shown) with the expandable volume76 defined by the piston 70, the chamber 68 and the closure plug 72. Thehydraulic package associated with each module is disposed at the ends ofthe wires 12 in that module opposite from the element ends at which thehydraulic package of the next adjacent modules are disposed. In otherwords, the chambers 64 and 68 are alternately disposed about thecircumference of each of the rim portions 40 and 46. Each of the firstchambers 64 is hydraulically paired with a next adjacent chamber 76. Thepiston 66, as illustrated in FIG. 4, is associated with a first group ofwires 12 defining a module and is hydraulically connected to theadjacent chamber 68 defined by the closure plug 72 in the rim portion40. However, the same hydraulic package for the group of wires 12 on theleft-hand portion of FIG. 4 are disposed in the lower rim portion 46.Thus, the hydraulic packages associated with the adjacent modules aredisposed at opposite ends of the wires 12.

Similarly, the cam follower assemblies 62 of alternately disposed frommodule to module so that the cam follower assemblies 62 of alternatemodules react with the upper cam 14 and the remaining cam followerassemblies 62 of the remaining modules react with the second or lowercam 16. In other words, half of the cam follower assemblies 62 extendradially from the upper rim portion 42 whereas the remaining camfollower assemblies 62 extend radially inwardly from the lower rimportion 46.

Each of the modules includes a fixed plate 78 fixedly connected to therim portion 40 or 46 of one of the wheels and a movable plate 80 movablyattached to the rim portion 40 or 46 of the other of the wheels. Eachmodule includes a plurality of wires 12 extending between a fixed plate78 and a movable plate 80 with the ends thereof connected to therespective fixed and movable plates 78 and 80. The fixed plates 78 areimmovably connected to the rim portions 40 and 46 by bolts 82 havingnuts 84 thereon. The bolts 82 extend through holes in the plate 78 suchas the hole indicated at 86 in FIG. 8 and through one of the rimportions 40 or 46 for securing the plate 78 in a fixed position relativeto the rim portion 40 or 46 to which it is connected.

The movable plates 80 are connected to the first ends of guide pins 88.The guide pins 88 extend through the rim portion 40 or 46 of one of thewheels to second ends which have the nuts 90 disposed thereon. Eachguide pin 88 is slidably supported in bushings of bearing material 92whereby the plates 80 may move as the pins 88 slide in the bushings 92.

Connecting means including the arms 94, the blocks 96 and threadedfastener assemblies 98 interconnect the second ends of the guide pins 88and the associated first piston 66 for creating hydraulic pressure inthe volume 74 of the chamber 64 in response to the unstraining, i.e.,shortening, of the wires 12.

A piston rod 100 is connected by a bolt 102 to each of the secondpistons 70 at one end and includes a U-shaped yoke 104 at the other endfor rotatably supporting the roller defining the cam follower 62. Eachcam follower 62 rollingly engages one of the axially extending surfaces20 and 22 of the cams 14 and 16. The rollers 62 are supported by theyokes by means of bolts 105. Guide shields 106 are disposed on eitherside of the cams 14 and 16 respectively and are maintained in positionby the clamping action of the bolts 105. The guide shields 106 engageopposite faces of the cams 14 and 16. The rods 100 extend throughbearing sleeves 107, the bearing sleeves 107 slidably supporting thepiston rods 100 for radial movement.

It will be noted that the first hydraulic piston 66 has a larger areasubjected to hydraulic fluid pressure than the second hydraulic piston70. The hydraulic pistons are in fluid communication with one anotherthrough a fluid communication means as the expandable volume 74 is influid communication through a hydraulic line or conduit with theexpandable volume 76. It will be noted that the first hydraulic pistons66 move axially or parallel to the central axis 18 whereas the hydraulicpistons 70 move radially relative to the central axis 18. This allowsthe wires 12 to extend axially and the cams 14 and 16 to projectradially with a resultant compactness.

The support means 26 which is defined by a housing includes or defines afirst compartment 108 extending in an arc circumferentially about thecentral axis 18 for a first portion of the circumferential periphery andis divided or separated from a second compartment 110 by a wall 112, thesecond compartment 110 extending in an arc about the central axis 18 foranother adjacent portion of the periphery thereof. The compartment 108is additionally defined by the wall 28 which has an outwardly disposedportion 114 with axially extending nozzle slots or passages 116extending radially through the walls 114 from each compartment towardthe wires 12. The housing also includes a plurality of inlets 118 withtop and bottom inlets 118 associated with each respective compartment108 and 110, it being appreciated that the housing is divided by fourradially extending walls 112 into four compartments. Two of thecompartments 108 or 110 would be for either hot or cold fluid (such aswater) with the hot or cold compartments 108 or 110 being diametricallyopposed. The housing also includes four outlets 120 with each outlet 120being associated with one of the compartments 108 or 110. The housing 26also includes an outer wall 117 disposed radially about the carriagemeans 24 with the fluid outlets 120 extending radially from the outerwalls 117 to exhaust the fluid from the housing. The section shown inFIG. 3 is illustrated in line 3--3 of FIG. 2, but the outlet 120 is notshown along the section line 3--3 of FIG. 2; however, it has been addedfor purposes of clarity to show the relative position of the outlets 120if the section line 3--3 passed through an outlet 120. Thus, hot andcold fluid may pass through the compartments 108 and 110 and radiallythrough the wires 12 to heat and cool the wires 12 as they move orrotate about the central axis 18.

The assembly also includes an output shaft 122 rotatably supported in ahousing portion 124 through appropriate bearings for rotating an outputmember 126. A power take-off means for transmitting the rotationalmovement of the carriage means to the output shaft 122 is included andcomprises a beveled ring gear 128 bolted by the studs 130 to the hubportion 38 of the upper wheel and a meshing beveled pinion gear 131which is attached to one end of the output shaft 122.

The assembly also includes stress limiting means for limiting the strainof each respective wire element 12 during the phases whereby the strainupon each wire element 12 may be greater during the first phase thanduring the second phase. The first phase occurs while the elements 12are at a relatively cool temperature below the transition temperatureand the second phase occurs while the elements 12 are at a relativelywarm temperature above the transition temperature. The strain is thepercent of elongation of each element wire 12 during the first phasewhen at the relatively cool temperature and is greater than the strainor percent of elongation of each element wire 12 during the second phasewhen the element is at the relatively warm temperature. The stressapplied to each element wire 12 in reacting with the reaction means isgreater during the second phase at the warmer temperature than is thestress subjected to each wire during the first phase wherein thetemperature is relatively cool. The stress limiting means allows work tobe efficiently extracted from the material by allowing the strain orpercent of elongation of the wires 12 to be greater at the relativelycold temperature than at the relatively warm temperature. The stresslimiting means is disposed between the cams 14 and 16 defining thereaction means and the wire elements 12 so that the cams 14 and 16 reactwith the wire elements 12 through the stress limiting means.Specifically, the stress limiting means includes a plurality of coilsprings 132 which allow a lost motion between the cams 14 and 16 and thewire elements 12. More specifically, each spring 132 reacts betweenassociated wire elements 12 and the plates 78 and 80 of the carriagemeans 24. Each of the plates 78 and 80 are part of the carriage meansand include a plurality of counterbores 134. One of the coil springs 132is disposed in each of the counterbores 134. Further, each of the plates78 and 80 of the carriage means 24 includes a recess 136 in the bottomof each counterbore 134. There is also included a connecting meanscomprising a first plug 138 for interconnecting the wires 12 and thecoil springs 132. The plugs 138 are disposed and retained in the top ofeach of the coil springs 132 and the wires 12 extend through the plugs138 to be retained thereto. Specifically, the plugs 138 have passagestherethrough with enlarged cavities 140 at one end with the wires 12extending through the passages and having enlarged heads 142 disposed inthe cavities. There is also included anchoring means comprising thesecond plugs 144 disposed in the recesses 136 with the wires 12extending through passages and the plugs 144 to enlarged cavities at theend of the passages with the wires having enlarged heads 148 disposed inthe cavity 146. The wires 12 have identical ends in that the heads 142and 148 are identical. Thus, each coil spring 132 is disposed about twoof the wires 12 and each anchoring plug 144 is connected to wires 12.The wires extend through passages in the plates 78 and 80 and thenthrough the respective plugs 138 and 144.

The anchoring plugs 144 are spaced longitudinally of the wires 12 fromthe adjacent coil springs 132 at the same ends of the wires 12 so thatthe anchoring plugs 144 overlap two adjacent coil springs 132 in adirection transverse to the longitudinal axis of the wires 12, as bestillustrated in FIG. 8. In other words, the bottom of each coil spring132 overlaps at least one anchoring plug 144 and overlaps two suchanchoring plugs 144, as illustrated.

The springs 132 defining the stress limiting means are disposed betweenonly one end of each of the wires 12 and the plates 78 or 80 of thecarriage means for limiting the strain of the elements as stress istransmitted between the wire elements 12 and the cams 14 and 16. Inother words, the springs 132 are disposed only at first ends of half ofthe wire elements 12 and are disposed only at the opposite or secondends of the remainder of the wire elements 12 thereby compacting thearea in which the wire elements 12 are disposed. Said another way, thecoil springs 132 are alternately disposed at the first and second endsof the wire elements 12.

OPERATION

The metal of which the wire elements 12 is made may be alloyed so thatthe transition temperature varies over a wide range. Depending upon thetransition temperature of the metal in the elements 12, water warmedabove the transition temperature is fed into the assembly through fourof the inlets 118 so as to fill two diametrically opposite pairs of thechambers 108 or 110 while colder water below the transition temperatureis fed through four inlets 118 so as to fill the other two pairedcompartments 108 or 110 with cold water. It will be appreciated thatcold water is subjected to two pair of inlets 118 which arediametrically opposed while warm water is supplied to the other two pairof inlets 118 which are diametrically opposed and 90 degrees apart fromthe inlets for the cold water, as there are two cold water compartments108 or 110 and two warm water compartments 108 or 110. The water passesthrough the compartments 108 and 110 out through the nozzle slots 116and radially through each group of wires 12 in each module and out theoutlets 120.

When the wires 12 are subjected to the relatively cool temperature belowthe transition temperature they become relatively soft and are subjectedto a force resulting in a stress upon the wires 12. While the wires 12are in the relatively cool state, the cam followers 62 are moving towardthe high point of the cams so as to force the pistons 70 toward theposition illustrated in cross section in FIG. 7 as the carriage means 24is rotating in a clockwise direction as illustrated in FIG. 7. Suchmovement contracts the volume 76 forcing hydraulic fluid to theunderside of the associated piston 66 to increase the volume of thechamber 74 and move the piston 66 upwardly, as viewed in FIG. 4. Suchmovement of piston 66 is transferred through the guide pins 88 to movethe movable plate 80 upward to stretch or elongate the wires 12, thusapplying a stress to the wires 12. Such a stress results in strain whichis the change in length of the wires 12 as a result of being subjectedto stress. Strain is described in physics formula as the change orelongation in length of the wire 12 divided by the original length ofthe wire 12. As related hereinbefore, the permissible strain of thewires 12 while in the relatively cool state is greater than thepermissible strain of the wires while in the warm state.

Once a cam follower 62 passes the high point and the cam begins movingdown the cam, as would the roller cam follower 62 shown in the middle ofFIG. 7 as it moves clockwise, the wires 12 are subjected to relativelywarm water passing out through the slots 116 from one of thecompartments. Thus, the wires 12 are heated to a temperature above thetransition temperature and contract or shorten to return to theiroriginal length. In so doing, the movable plate 80 is moved downwardlyto move the piston 66 downwardly forcing hydraulic fluid to theexpandable volume 76 in the next adjacent piston which drives the rod100 and the associated cam follower 62 against the cam surface whichresults in a force vector to rotate the carriage means 24. It will beappreciated that less energy is required to elongate the wires in therelatively cool state than results from the wires contracting in therelatively warm state whereby the wires convert heat to mechanicalenergy during the unstraining thereof.

Since there are two quadrants for each of the warm and cool liquids thewires are strained and unstrained twice during each revolution.

Since it takes less force to strain the wires in the cold state, thesprings 132 are selected such that there is minimal compression of thesprings 132 when the wires 12 are being strained and elongated in therelatively cold state. However, when the wires 12 move to the relativelywarm state above the transition temperature there is a great deal moreresultant stress yet the wires have less permissible strain in the warmstate because the effective modulus of elasticity of the elements 12changes between the relatively cool and relatively warm temperatures.The springs 132 are stress limiting means for limiting the strain in theelements 12 during the warm state as the springs 132 are compressed toprevent the stress in the wires 12 from exceeding the predeterminedlimit and thereby preventing the permissible strain of the wires frombeing exceeded.

It will be appreciated that the strain in a material having a shapememory characteristic may result from various different stresses and thestress limiting means for limiting the strain (whereby the strain may begreater in the relatively cold state than in the relatively warm state)may take forms other than the springs illustrated herein. For example,the stress limiting means may be a form of a slip clutch, elastic memberor other lost motion device which will allow the most efficientutilization of the material by allowing a greater strain in therelatively cold state, which is the phase in which the stress applied tothe material results in more strain, than the strain allowed in thematerial in the second phase when the material is unstraining and doingwork.

The fluid utilized to provide the different temperatures is preferably aliquid but may also take the form of a gas or gases or a combination ofliquid and gas.

Another feature of the embodiment illustrated is that it is symmetricalabout a plane extending diametrically through the assembly (as throughthe high points of the cams) and therefore may rotate in eitherdirection once started in that direction. In other words, the directionof rotation may depend upon the direction in which the assembly isinitially rotated for start-up.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology which has been used is intended to bein the nature of words of description rather than limitation.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A thermal energyscavenger assembly comprising: at least one temperature-sensitiveelement made of material which exhibits shape memory due tothermoelastic, martensitic phase transformation; reaction means reactingwith said element for applying a stress to said element to strain saidelement during a first phase and for responding to the unstraining ofsaid element during a second phase; and stress limiting means forlimiting the strain of said element during said second phase whereby thestrain upon the element may be greater during the first phase thanduring the second phase.
 2. An assembly as set forth in claim 1 whereinsaid phase transformation occurs as said element passes through atransition temperature between a relatively cool temperature below thetransition temperature and a relatively warm temperature above thetransition temperature, said first phase occurring while said element isat said relatively cool temperature, said second phase occurring whilesaid element is at said relatively warm temperature, said strain in saidelement during said first phase being greater than said strain in saidelement during said second phase, and said stress applied to saidelement in reacting with said reaction means being greater during saidsecond phase than during said first phase.
 3. An assembly as set forthin claim 2 wherein said stress limiting means is disposed between saidreaction means and said element so that said reaction means reacts withsaid element through said stress limiting means.
 4. An assembly as setforth in claim 3 wherein said stress limiting means allows lost motionbetween said reaction means and said element.
 5. An assembly as setforth in claim 4 wherein said stress limiting means includes a spring.6. An assembly as set forth in claim 4 wherein said reaction meansincludes cam means for reacting with said element during the strainingand unstraining thereof, and including support means supporting said cammeans and said element for allowing relative movement between saidelement and said cam means to extract energy as said element reacts withsaid cam means through said stress limiting means during the unstrainingof said element in said second phase.
 7. An assembly as set forth inclaim 4 including carriage means supporting said element for allowingsaid element to be placed in tension while reacting with said reactionmeans.
 8. An assembly as set forth in claim 7 wherein said stresslimiting means includes an elastic member interacting between saidelement and said carriage means so that the stress applied to saidelement by said reaction means is transmitted through said elasticmember.
 9. An assembly as set forth in claim 8 wherein said elementcomprises a wire.
 10. An assembly as set forth in claim 9 wherein saidelastic member is a coil spring disposed about one end of said wire andreacting therewith so that said spring is compressed during saidunstraining of said spring in said second phase.
 11. An assembly as setforth in claim 10 including a plurality of said wires and a plurality ofsaid springs.
 12. An assembly as set forth in claim 11 wherein saidreaction means includes cam means for reacting with said wires duringthe straining and unstraining thereof, and including support meanssupporting said cam means and said carriage means for allowing relativemovement between said cam means and said wires supported by saidcarriage means to extract energy as said wires unstrain and shorten inlength as they compress said springs in reacting with said cam meansduring said second phase.
 13. An assembly as set forth in claim 2including a plurality of said elements with each element comprising awire in tension by forces applied through said stress limiting means,and said stress limiting means comprises spring means having a springrate for remaining substantially immovable to elongate said wires insaid first phase and movable in response to the increased stress duringsaid second phase to prevent the stress in the wires from exceeding apredetermined limit in said second phase thereby preventing thepermissible strain of the wires from being exceeded in said secondphase.